The present invention relates to mass spectrometry and, more particularly, to the detailed characterization of individual particles by mass spectrometry.
Particle analysis is important in a wide variety of industrial processes including the fabrication of high performance semiconductor and optoelectronic devices. As feature sizes on semiconductor devices shrink, the size of particles that causes defects also decreases. On today""s advanced devices, particles as small as 0.1 micrometers can cause yield reducing defects. In the near future, as feature sizes on devices become smaller and smaller, particles as small as 0.02 micrometers will result in significant defects. Such particles can come from several sources including delaminating films, broken wafers, atmospheric dust, and the vacuum processes used for the deposition and etching of thin films, especially plasma processes. Analysis of the chemical composition of the particles is an important step in finding the root cause of particle contamination.
There now exist highly accurate techniques for detecting and analyzing sub-micron particles. Such techniques are described in U.S. Pat. No. 5,382,794 issued to S. W. Downey et al. on Jan. 17, 1995 and U.S. Pat. No. 5,631,462 issued to one of the present inventors W. D. Reents, Jr. on May 20, 1997, both of which are incorporated herein by reference. In essence, the particles are entrained within a gas stream, fragmented and ionized by a laser beam (xe2x80x9claser ablationxe2x80x9d), and the chemical nature and concentration of the species within the particle is determined by mass spectrometry. This approach permits limited real-time characterization of particles as small as 1 nm in diameter.
Although such previous efforts are useful in many situations, they are not capable of providing detailed information concerning the size and shape of a particle nor can they provide information concerning the relative positions of elements within the particles.
We have recognized that, while the ""794 and ""462 patents provide exemplary methods for obtaining limited types of particle-related information, there remains a need to provide a method for obtaining more detailed information such as, for example, the size, shape, and relative position of elements within particles. Such detailed information greatly enhances the ability to identify the origination point and nature of the particles in question by providing the crucial information needed, for example, to identify and isolate the source of the particles.
Therefore, we have invented a method for determining the shape and size of particles and the distribution of bulk constituent elements within those particles. Specifically, in a first aspect of the invention, a particle is ionized using a laser with sufficient power to fragment and ionize substantially all elements of the particle. The resulting ions are sampled by a mass spectrometer useful in identifying the bulk chemical composition of the source particle. The time-varying intensity of the mass-resolved ions are measured as they strike a detector. The integrated intensity over time for each ion mass is related to the total number of ions that existed in the source particle prior to ablation. The temporal width of a mass-resolved ions"" intensity is related to the diameter of the original particle.
In another aspect of the invention, the shape and the relative position of each constituent element of a particle can be determined. In accordance with this aspect, as described above, substantially all the elements of a particle are ionized and the ions of each particle are identified by mass spectrometry. The intensity of the ions impacting a detector over time is measured for each element in order to develop a series of intensity vs. time plots corresponding to the ions of each element within the source particle. The plots of intensity versus time represent the one-dimensional spatial distribution of the ions just prior to impact with the detector. Since the final spatial distribution of ions is related to the initial one-dimensional distribution of elements within the particle, the shape of the intensity versus time plot itself is directly related to the one-dimensional shape of the particle source and the distribution of each of its elements within the source particle. Thus, by comparing the plots of the different elements relative to each other, the cumulative shape of the original particle with those elements can be determined.
In yet another aspect of the invention, the one-dimensional to three-dimensional shape and relative position of the particle and its constituent elements can be determined by photographing a phosphor screen detector that emits light at those points where ions impact the surface of the detector. This image reflects a two-dimensional (defined by the plane of the detector) image of the spatial location of the ions as they arrive to the detector. A high-speed camera captures images of the resulting light pattern at closely-spaced successive moments in time. These images represent the cross section of the original particle at each moment in time. Successive images in time represent the third dimension of the spatial location of the mass-resolved ions in a manner similar to the previous description of the invention. By combining the multiple time-resolved cross section images that are thus captured, a three-dimensional image of the elemental distribution within the original particle is obtained. By overlaying the three-dimensional images for each element in the particle, the complete shape of the original particle can be obtained.